Why is my free testosterone level low but the the total is normal?

Free T hard to do. Generally free testosterone is quite a difficult test to do accurately in the labs. Clinical judgement, your clinical presentation, and two separate low testosterone values are necessary to really clarify your diagnosis.
Confusing area. "normal" values for free and total testosterone are all over the map. Depending on the lab director, t nrml ranges from 175 to 1100.! go figure. It drives patients and doctors crazy when trying to make sense of "low t". The fda considers a t less than 300 with symptoms as treatable. Free t assays are variable from lab to lab wheras total t is pretty standard. You should always repeat abnormals.